Current wireless communication technologies, such as Long Term Evolution (LTE) Advanced, adopt orthogonal frequency division multiplexing (OFDM) as the waveform candidate for the air interface. However, there are some key limitations, such as high peak to average power ratio (PAPR), low spectral efficiency, and strict orthogonality requirements, which significantly inhibit the performance of OFDM. To address the drawbacks of OFDM and satisfy the requirements of future communication networks, new waveforms are being proposed both in the literature and among standardization bodies. On one hand, there are proposals to preserve OFDM as the fundamental waveform for 5G by finding proper solutions to some of its shortcomings, making the new waveform backward compatible with existing technologies. On the other hand, different multicarrier schemes other than OFDM, have been intensely studied in the literature as alternative waveform. Some waveforms under consideration in both industry and academia can be classified into three main categories: complex orthogonal, real orthogonal, and non-orthogonal waveforms.
In this post we will give a brief overview of some complex orthogonal waveforms. In a future post, we will discuss the two other classes of waveforms.
Complex Orthogonal Waveforms
The structure of a conventional OFDM is shown in Figure 1. To achieve communication between a transmitter and a receiver, the input data symbols at the transmitter are mapped onto the appropriate constellation using a modulation scheme such as QAM. The resultant signal is converted into a parallel stream, of size equal to the fast Fourier transform (FFT), and fed into an inverse FFT (IFFT) block, to be converted to the time domain. A cyclic prefix is then inserted by copying a specific period from the tail of each symbol and appending it to the beginning of that symbol. This serves as a guard interval to extend the symbol duration and thereby reduce inter-symbol interference. At the receiver, the demodulated data symbols can be obtained by reversing the processing at the transmitter. CP-OFDM employs rectangular transmit and receive prototype filters.
An alternative OFDM-based waveform that has received attention within 3GPP is the f-OFDM. At the transmitter of f-OFDM system, a filter is employed after CP-OFDM processing to deal with the OOB emissions. Filtering is done over an entire band which improves the performance of CP-OFDM in applications that require asynchronous transmissions. A similar filter is employed at the input to the CP-OFDM receiver, to deal with inter-user interference. A sync pulse multiplied by a Hann window is usually employed as the filter.
UFMC combines CP-OFDM and f-OFDM, by applying filtering to sub-bands instead of the complete band, as is the case for f-OFDM. The filtering operation for the sub-band wise processing is based on a Dolph-Chebyshev window. However, in UFMC filtering is only done at the transmitter. The demodulation at the receiver is achieved by first applying a window to the received signal, followed by FFT double the size of the IFFT processing at the transmitter side. The use of double FFT at the receiver of UFMC enable the demodulation of data without the use of CP.